The present invention relates to an electromagnetic flowmeter in which a measuring pipe is made of a ceramic material and, more particularly, to an improvement of its electrode structure.
A conventional electromagnetic flowmeter utilizes a Faraday's law of electromagnetic induction and converts a flow rate of a conductive fluid which passes through a measuring pipe into an electrical signal. Conventional electromagnetic flowmeters having various structures are available. For example, a typical measuring pipe of an electromagnetic flowmeter of this type is a stainless steel pipe in which an insulating lining is formed on its inner surface. In recent years, a ceramic measuring pipe made of alumina (Al.sub.2 O.sub.3) serving as a nonconductive material is developed and popular due to advantages in which the pipe has corrosion and abrasion resistance and is not deformed at high temperatures.
A conventional electromagnetic flowmeter using this ceramic measuring pipe employs an insertion type electrode structure mounted in a measuring pipe such that liquid contact ends are exposed inside the measuring pipe. An electrode having a large-diameter liquid contact end exposed in the measuring pipe is inserted in a corresponding electrode insertion hole formed in a wall of the measuring pipe from the inside of the measuring pipe. An electrode shaft portion extending outside the measuring pipe is held outside the measuring pipe by means of a proper press member, a spring and the like.
However, in the conventional electrode structure wherein the insertion electrodes are fixed to the ceramic measuring pipe having the above arrangement, when an electrode is inserted from the inside of the measuring pipe and is fastened and fixed by the distal end portion of the electrode shaft portion with a press member, a spring, or the like, the wall portion of the measuring pipe is overloaded to undesirably damage the measuring pipe.
The present inventors made extensive studies to prevent this drawback. The present inventors found that the measuring pipe was damaged by a bending force and a tensile load acting on the wall portion of the measuring pipe due to a position difference between a force acting on the measuring pipe from the electrode liquid contact end inside the measuring pipe and an external force exerted by the above-mentioned press member or the like when the insertion electrodes are fastened and fixed on the outer side of the measuring pipe.
A ceramic material as a measuring pipe material has its mechanical properties such that it can withstand a compression load but cannot withstand a tensile load. For example, the strength of alumina or the like against the compression load is 200 kgf/mm.sup.2, while its flexural strength (about 1.7 times the tensile strength) is 300 kgf/mm.sup.2. Therefore, when the electrodes are fastened and fixed to obtain the electrode structure described above and the inner and outer points of forces with respect to the measuring pipe are offset from each other, a tensile load acts on the measuring pipe, thus damaging the measuring pipe. Strong demand has arisen for an electrode structure for solving the above problem. In particular, in this electrode mounting portion, liquid leakage or the like must be perfectly prevented, and the liquid contact end must be firmly connected to the press member. Therefore, a structure which can reduce the tensile load described above is required. In order to prevent the tensile load from acting on the measuring pipe, a point of force acting on each electrode liquid contact end inside the measuring pipe must be aligned with a point of force acting on the press member or the like attached to the corresponding electrode shaft portion outside the measuring pipe. However, high precision is required to arrange the respective parts so as to coincide the points of force with each other. In practice, a countermeasure for solving all these problems with a simple structure is desired.
In addition, a seal material such as a gasket must be fitted on each electrode mounting portion of the measuring pipe to assure perfect sealing.